1. Field of the Invention
This invention relates to a memory device formed of electrically rewritable and non-volatile memory cells, and specifically relates to a method of programming a non-volatile memory device with variable resistance elements used as memory cells.
2. Description of the Related Art
Recently, there is noticed and researched a resistive RAM (ReRAM), as one of non-volatile memory devices, which has an electrically rewritable and variable resistance element for storing a resistance value as data.
A variable resistance element used as a memory cell in a ReRAM is constituted to have a stacked structure of electrode/metal oxide/electrode. It is known that there are two kinds of operation modes in the variable resistance elements as follows: in one mode, the polarity of applying voltage is exchanged, so that a high resistance state and a low resistance state are exchanged in a memory cell; and in the other mode, applying voltage value and applying time thereof are controlled without exchanging the applying voltage polarity, so that a high resistance state and a low resistance state are exchanged. These two modes are referred to as a bipolar type and a unipolar (or non-polar) type, respectively.
To achieve a highly integrated memory cell array, it is preferred to use the unipolar type because it is possible to constitute a memory cell array by stacking variable resistance elements and rectifying elements such as diodes at the respective cross points of word lines and bit lines without transistors.
In case a cross-point type of memory cell array is formed without rectifying elements, cross-talk noises cause program disturbances on non-selected cells. To reduce the influence of the cross-talk disturbance, it has been provided such a method as to apply a program-compensating voltage to the non-selected cells after applying a program voltage (refer to JP-A2006-344349).
On the other hand, to prevent the cross-talk of the cross-point type of cell array, it is useful that a rectifying element such as a diode is serially coupled to a variable resistance element (for example, refer to U.S. Pat. No. 6,831,854).
Further, in the cross-point type of cell array, both of data write and data erase may be performed with the same polarity voltage as each other, and a short pulse is used for data write while a long pulse is used for data erase. These are disclosed by Y. Hosoi et al, “High Speed Unipolar Switching Resistance RAM(RRAM) Technology” IEEE International Electron Device Meeting 2006, Technical Digest, p. 793-796.